eeepc-wmi: add cpufv sysfs documentation

Based on cpufv text from sysfs-platform-eeepc-laptop that
has almost same behavior.
Signed-off-by: NChris Bagwell <chris@cnpbagwell.com>
Signed-off-by: NMatthew Garrett <mjg@redhat.com>
Acked-by: NCorentin Chary <corentincj@iksaif.net>

Documentation/ABI/testing/sysfs-platform-eeepc-wmi

+What:		/sys/devices/platform/eeepc-wmi/cpufv
+Date:		Oct 2010
+KernelVersion:	2.6.37
+Contact:	"Corentin Chary" <corentincj@iksaif.net>
+Description:
+		Change CPU clock configuration (write-only).
+		There are three available clock configuration:
+		    * 0 -> Super Performance Mode
+		    * 1 -> High Performance Mode
+		    * 2 -> Power Saving Mode